Are you able to run On-Demand report/task if no selections is done?
If so, the problem can be due to a character that is passed to NPrinting with a selection.
My suggestion is that you open a case with the QlikView support team.
Could you attach a complete environment to replicate the error (NPrinting file, template, scrambled\reduced QVW and the dashboard you use for the On-Demand)?
We will check which is the character causing the error message and fix the problem.
Kindly, verify be sure that you are running the latest NPrinting release, the problem may already be resolved.